Somehow, even changing your weight from 115 Kg (253.5 lbs) to 57.5 Kg (126.7 lbs) does not make your numbers add up to easily planing at the windspeeds you are talking about.
If you changed your sail size/windspeed to 10-12 knots on a 6.5-7.5 m2 rig, 12-16 knots on your 5.5, and 16-20 knots on your 4.7 m2 rig I could see you being powered up (at 126 lbs/57.5 Kg) enough to complete a jibe in the upper wind range of each sail.
Are you trying to learn a fully planing carve jibe, or something else.
The difference between a 40 cm Drake Free Slalom and a 37 cm Drake Freeride fin is fairly insignificant at 14 knots windspeed unless you have alot of chop to deal with.
If lots of chop, they CC is correct the more "curvy" planform of the 37 cm Freeride will be a bit more forgiving, but what you gain in higher speed "forgivingness" you will lose in your ability to get on plane earlier and you will lose a bit of upwind capability as well.
Hope this helps,